Bipartite-ness under smooth conditions

Abstract

Given a family $\mathcal{F}$ of bipartite graphs, the {\it Zarankiewicz number} $z(m,n,\mathcal{F})$ is the maximum number of edges in an $m$ by $n$ bipartite graph $G$ that does not contain any member of $\mathcal{F}$ as a subgraph (such $G$ is called {\it $\mathcal{F}$-free}). For $1\leq \beta<\alpha<2$, a family $\mathcal{F}$ of bipartite graphs is $(\alpha,\beta)$-{\it smooth} if for some $\rho>0$ and every $m\leq n$, $z(m,n,\mathcal{F})=\rho m n^{\alpha-1}+O(n^\beta)$. Motivated by their work on a conjecture of Erd\H{o}s and Simonovits on compactness and a classic result of Andr\'asfai, Erd\H{o}s and S\'os, in \cite{AKSV} Allen, Keevash, Sudakov and Verstra\"ete proved that for any $(\alpha,\beta)$-smooth family $\mathcal{F}$, there exists $k_0$ such that for all odd $k\geq k_0$ and sufficiently large $n$, any $n$-vertex $\mathcal{F}\cup\{C_k\}$-free graph with minimum degree at least $\rho(\frac{2n}{5}+o(n))^{\alpha-1}$ is bipartite. In this paper, we strengthen their result by showing that for every real $\delta>0$, there exists $k_0$ such that for all odd $k\geq k_0$ and sufficiently large $n$, any $n$-vertex $\mathcal{F}\cup\{C_k\}$-free graph with minimum degree at least $\delta n^{\alpha-1}$ is bipartite. Furthermore, our result holds under a more relaxed notion of smoothness, which include the families $\mathcal{F}$ consisting of the single graph $K_{s,t}$ when $t\gg s$. We also prove an analogous result for $C_{2\ell}$-free graphs for every $\ell\geq 2$, which complements a result of Keevash, Sudakov and Verstra\"ete in \cite{KSV}.